docs: def label_total_duration(self):
          """
          Return for each distinct label value the total duration of all occurrences.
  
          Returns:
              dict: A dictionary containing for every label-value (key)
                    the total duration in seconds (value).
  
          Example:
              >>> ll = LabelList(labels=[
              >>>     Label('a', 3, 5),
              >>>     Label('b', 5, 8),
              >>>     Label('a', 8, 10),
              >>>     Label('b', 10, 14),
              >>>     Label('a', 15, 18.5)
              >>> ])
              >>> ll.label_total_duration()
              {'a': 7.5 'b': 7.0}
          """
  
          durations = collections.defaultdict(float)
  
          for label in self:
              durations[label.value] += label.duration
  
          return durations